When only the option 'on-click-right' was set and no other 'on-click'
option than the taskbar module wouldn't register for click events and
hence those events were handled by the generic AModule::on-click code.
This code would try to start a shell with the specified command, which
wouldn't make any sense in this circumstances.
The taskbar code falsely checked for the 'on-click-left' option instead
for the 'on-click-right' when deciding to register for click events.
